Electrolytic bath to be used for electrolytically anodizing aluminum or aluminum alloy to form a colored oxide coating and method for anodizing said metal

ABSTRACT

ANODIZING ELECTROLYTIC BATH FOR PRODUCING COLORED ALUMINUM OR ALUMINUM ALLOY THE BATH BEING AN AQUEOUS SOLUTION CONTAING 2.4 TO 40% BY WEIGHT OF CRESOLSULFONIC ACID AND 0.05 TO 3% BY WEIGHT OF SULFURIC ACID OR A METAL SULFATE IN AN AMOUNT EQUIVALENT TO THE CONCENTRATION OF SAID SULFURIC ACID. THE AMOUNT OF THE CRESOLSULFONIC ACID ADDED CAN BE DECREADED TO 0.5 TO 40% BY WEIGHT BY ADDING 0.5 TO 5.0% BY WEIGHT OF SULFOSALICYLIC ACID. METHODS FOR PRODUCING COLORED OXIDE COATING ON THE SAID METAL BY MEANS OF THE ABOVE DESCRIBED BATH ARE ALSO DISCLOSED.   D R A W I N G

HIROSHL NAKAZATO ELECTROLYTIC BATH TO BE USED FOR ELECT Jan. 11,, 1972 ETAL ,634,2fl4

ROLYTICALLY ANODIZING ALUMINUM OR ALUMINUM ALLOY TO FORM A COLORED OXIDE COATING AND METHOD FOR ANODIZING SAID METAL Flled July 18, 1969 "United States Patent Office 3,634,214 Patented Jan. 11, 1972 3,634,214 ELECTROLYTIC BATH TO BE USED FOR ELEC- TROLYTICALLY ANODIZING ALUMINUM OR ALUMINUM ALLOY TO FORM A COLORED OX- IDE COATING AND METHOD FOR ANODIZING SAID METAL Hiroshi Nakazato, Shimizu, and Masayoshi Yokoyama, Toshihiro Nagano, and Kazuyoshi Kaneda, Shizuoka, Japan, assignors to Riken Denka Kogyo Co., Ltd., Magarikane, Shizuoka, Japan Filed July 18, 1969, Ser. No. 842,959 Claims priority, application Japan, July 18, 1968, 43/50,442, 43/ 50,443; Oct. 19, 1968, 43/75,839; Apr. 2, 1969, 44/25,068

Int. Cl. C23b 9/02 US. Cl. 204-58 8 Claims ABSTRACT OF THE DISCLOSURE Anodizing electrolytic bath for producing colored aluminum or aluminum alloy the bath being an aqueous solution containing 2.4 to 40% by weight of cresolsulfonic acid and 0.05 to 3% by weight of sulfuric acid or a metal sulfate in an amount equivalent to the concentration of said sulfuric acid. The amount of the cresolsulfonic acid added can be decreased to 0.5 to 40% by weight by adding 0.5 to 5.0% by weight of sulfosalicylic acid. Methods for producing colored oxide coating on the said metal by means of the above described bath are also disclosed.

The present invention relates to electrolytic baths to be used for electrolytically anodizing aluminum or aluminum alloy to form a colored oxide coating and also to methods for anodizing said aluminous substrate by means of the electrolytic bath to form a colored oxide coating having excellent abrasion resistance and weather resistance on said substrate.

Recently, building modes have been modernized and aluminous building materials have been utilized broadly. Particularly various colored aluminous materials having excellent abrasion resistance and weather resistance are presently required.

Heretofore, a method for forming a colored oxide coating on one of the surfaces of an aluminous material by subjecting the aluminous material to electrolyse in an electrolytic bath to color the aluminous material, said material serving as an anode, that is, an anodizing process,

has been used and various electrolytic baths have been used for the anodizing treatment. I

When an aqueous solution of sulfuric acid is used as an electrolytic bath in the above described anodizing process, the color tone of the oxide coating is usually gray and therefore, in order to color in the desired color tone, the above oxide coating is further colored by an organic or inorganic dyestuff. However in order to color in the same color tone it is necessary to strictly control the thickness of the oxide coating, the temperature and concentration of the dyestuff solution, the immersing time and the like in order to maintain them constant. Furthermore, the dyestuif lodges in the pores in the oxide coating and therefore the abrasion resistance is not suflicient and the color matter of the dyestuff fades with sun-light and the weather resistance is poor.

In order to avoid these defects, a method for forming a rigid, tight oxide coating has been proposed, which method comprises maintaining the temperature of the electrolytic bath at a low temperature, lower than room temperature, when using an electrolytic bath which is an aqueous solution of sulfuric acid, to prevent dissolution of the aluminous substrate, but in this case an expensive cooling installation is necessary.

An electrolytic bath which is an aqueous solution of oxalic acid or malonic acid has been disclosed in Japanese patent application publication No. 1,732,/ 65 and in this case the color tone of the oxide coating is limited to yellow or yellowish brown and therefore disadvantageously, it is impossible to color in any desired color.

In addition, electrolytic baths comprising sulfosalicylic acid mixed with sulfuric acid or metallic sulfates or an aqueous solution of sulfophthalic acid mixed with sulfuric acid or Water soluble sulfate or bisulfate have been disclosed in Japanese patent application publication No. 22,259/ 61. These electrolytic baths can provide an excellent oxide coating and a broad range of color tones, but sulfosalicylic acid and sulfophthalic acid are special chemicals and therefore the electrolytic bath becomes expensive.

Moreover, these methods require a high voltage, the concentration of the bath is high and bath is degraded rapidly and these electrolytic baths are not economically satisfactory.

Particularly, When aluminous materials are colored in light color, it is necessary to effect the electrolysis for a short time and consequently the obtained oxide coating is thin and has substantially no practical value.

Accordingly, an object of the present invention is to provide electrolytic baths capable of economically forming oxide coatings having the required colors and improved abrasion resistance and weather resistance on aluminous materials.

A further object of the present invention is to provide a method for forming colored oxide coatings having improved abrasion resistance and weather resistance an aluminous materials by anodizing the aluminous materials using the above described electrolytic baths.

Other objects, advantages and features of this invention will be readily apparent to those skilled in the art from the following description.

We have conducted many investigations in order to avoid the above described defects and have found that a colored oxide coating having improved abrasion resistance and weather resistance can be obtained by using an electrolytic bath which is an aqueous solution containing cresolsulfonic acid and sulfuric acid or a metal sulfate in an amount equivalent to the concentration of said sulfuric acid or an aqueous solution containing cresolsulfonic acid,

' sulfosalicylic acid and sulfuric acid or a metal sulfate in an amount equivalent to the concentration of said sulfuric acid on the anodizing of aluminum or aluminum alloy, which serves as the anode in the electrolysis.

Namely, the first aspect of the present invention consists of an electrolytic bath which is an aqueous solution containing 2.4 to 40% by weight of cresolsulfonic acid and 0.05 to 3% by Weight of sulfuric acid or a metal sul fate equivalent to the concentration of said sulfuric acid. In the above electrolytic bath, when the content of cresolsulfonic acid is less than 2.4% by weight, the aluminous substrate becomes corroded and pores are formed on the substrate and the oxide coating is not substantially We have made many studies to prevent the formation of corroded pores occurring with a low content of cresolsulfonic acid and have found that the content of cresolsulfonic acid can be decreased to 0.5% by weight by adding a small amount of sulfosalicylic acid to the electrolytic bath. Since the content of cresolsulfonic acid can be decreased, the dissolution of aluminum is slight and also the generation of gaseous hydrogen is slight and therefore the bath is hardly degraded and even an electrolytic bath used for a long time can color similarly to a fresh bath.

The second aspect of the present invention consists in an electrolytic bath which is an aqueous solution contain- TABLE 1 Electrolyte, weight percent Sul- [uric Cresol- Genaeid, Sulfonlc Current Time erated peracid, density, minvoltage, Shape cent percent a./dm. utes volts Appearance 0. 6 1 2 30 35-20 Corroded pores are many. 0.6 2 2 30 35-28 Corroded pores are very w. .6 2.4 2 30 30-45 Yellowish brown A .6 2 30 5 A 0.6 10 2 30 0.6 2 O 0.6 30 2 30 30-45 do A 0.6 2 30 30-45 Light yellowish A brown. 0.6 2 30 30-45 Light white 0.6 2 30 30-45 ..do

NOTE.-=Excellcut; O= Good; A=Fair.

When the content of sulfuric acid is less than 0.05% by weight, the degree of dissociation is low and even if the voltage is increased, the oxide coating is not substantially formed on the aluminous substrate. On the other hand, if the content of sulfuric acid is more than 3% by weight, the degree of dissociation is too high and it is difficult to obtain an oxide coating having a desired color.

Such results have been confirmed by the following experiment.

Aluminum 1100 was anodized by varying the content of sulfuric acid as shown in the following Table 2 without varying the content of cresolsulfonic acid of 10% by weight, while the temperature of the electrolytic bath was maintained at 20 C. The obtained results are shown in the following Table 2.

ing 0.5 to 40% by weight, preferably 3 to 10% by weight of cresolsulfonic acid, 0.5 to 5.0% by weight, preferably 0.75 to 1.5% by weight of sulfosalicylic acid and 0.05 to 3.0% by weight, preferably 0.1 to 0.7% by weight of sulfuric acid or a metal sulfate in an amount equivalent to said content of sulfuric acid.

In this electrolytic bath, when the content of cresolsulfonic acid is less than 0.5% by weight, the aluminous substrate is corroded to cause pitting phenomenon and substantially no oxide coating is formed, while when the content is more than 40% by weight, the color tone of the coating decreases and becomes white as mentioned in the above described bath of the first aspect of the invention. When the content of sulfosalicylic acid is less than 0.5%

TABLE 2 Electrolyte weight percent o Cresolrent Gensul- Suldenerated ionic [uric sity, Time volaeid acid, a./ (mintage, Shape percent percent (1m. utcs) volts Appearance 10 0 2 30 35-30 Non-color 10 0. 05 2 30 35-30 Yellowish brown A 10 0.4 2 30 34-55 .do. 0 10 0.6 2 30 30-42 .do. 0 10 0.8 2 30 20-36 .do. O 10 1.0 2 30 26-31 do A 10 1. 6 2 30 24-29 Light yellowish brown A 10 2.0 2 30 21-27 Light yellow A 10 3.0 2 30 20-24 Light white A 10 4. 0 2 30 17-20 Non-eolor transparent.

NOTE.= Excellent; 0 Good; A= Fair.

Furthermore, we have found that when aluminum 1100 was anodized by means of an electrolytic bath containing 10% by weight of cresolsulfonic acid and various amounts of ferric sulfate equivalent to form 0.05 to 4.0% by weight of the sulfuric acid instead of the sulfuric acid, substantially the same results as shown in the above Table 2 were obtained.

by weight, it is impossible to prevent the pitting, while when the content is more than 5% by weight, it is impossible to obtain an excellent colored anodized coating economically at low voltage. Moreover, the limitation of the content of sulfuric acid or a metal sulfate is based on the reason explained in the above described bath of the first aspect of the invention.

The above described electrolytic baths of the present invention have the following advantages.

(1) The components of the electrolytic bath are cresolsulfonic acid and sulfuric acid or metal sulfates; or cresolsulfonic acid, sulfosalicylic acid and sulfuric acid or metal sulfates and therefore the electrolytic bath is inexpensive.

(2) The dissociation of the electrolytic bath is high and the voltage in the electrolysis is low, so that the required amount of electric power decreases and the anodizing treatment can be carried out without raising the temperature of the bath during such a treatment. Furthermore, the anodizing time is short and the amount of chemicals consumed is small.

(3) The colored oxide coating formed on the aluminum substrate has excellent abrasion resistance and weather resistance and a stable color tone.

(4) By changing the material of aluminum, oxide coatings having a broad range of color tones, such as yellowish brown, bronze color, dark black, etc. can be obtained.

The third aspect of the present invention is a method of producing colored oxide coating on an aluminum substrate, which comprises immersing the aluminous substrate in any of the above described electrolytic baths of the first and the second aspects of this invention and passing an electric current through the electrolyte with the aluminous substrate serving as the anode.

In this case, the conditions for the electrolysis are as follows. The concentrations of the electrolytic baths are the same as illustrated in the first and second aspects of the invention. When using any electrolytic bath, the amount of aluminum dissolved is 0.5-1.2 g./l., the temperature of the bath is to C., the voltage is 25 to 80 volts, the current density is 1 to 3 a./dm. and the treating time is 15 to 60 minutes. However in the shorter time, a satisfactory color can not be obtained.

By such an anodizing method, it is possible to obtain an excellent colored oxide coating having improved abrasion resistance and weather resistance but when it is desired to obtain an oxide coating of light color, the treat ment must be effected for a short time. However the oxide coating thus obtained is thin and can not be substantially used practically.

In order to obtain an oxide coating having a satisfactory thickness and a desired light color, we have made various investigations and found that beautiful light yellow or yellowish brown oxide coating having excellent abrasion resistance and weather resistance can be formed by electrolytically anodizing an aluminous substrate in an aqueous solution of sulfuric acid or oxalic acid to form an oxide coating having a desired thickness and then further anodizing the thus treated aluminous substrate in any of the above described electrolytic baths of the present invention for a short time.

The fourth aspect of the present invention is a method of producing a light color oxide coating which comprises firstly immersing an aluminous substrate in an aqueous solution of sulfuric acid or oxalic acid and passing an electric current through the electrolyte with the aluminous substrate serving as the anode to form an oxide coating on the aluminous substrate and then, without effecting a treatment for sealing the pores in the oxide coating, anodizing the thus treated aluminous substrate in any of the above described electrolytic baths of the present invention for a short time such as less than 10 minutes.

As mentioned above, in this aspect of the invention, the aluminous substrate is firstly anodized in an electrolytic bath which is an aqueous solution of sulfuric acid or oxalic acid to form a porous oxide coating and then the formed porous oxide coating is further anodized in any of the above described electrolytic baths of the present invention to form a thin colored oxide coating having a light color on the above described porous oxide coating.

The conditions of electrolysis in the above described first step are as follows:

The conditions of electrolysis in the above described second step follow the conditions described in the above described third aspect of this invention, except that the treating time is 5 to 10 minutes. The thus obtained oxide coating is thoroughly washed with water and is subjected to a treatment for sealing the pores by 4 kg./m. of steam pressure.

Furthermore, we have found that aluminum or aluminum alloy anodized in any of the electrolytic baths of the present invention has satisfactory properties but the obtained colors are dark yellowish brown, grayish brown, amberish gray and the like and are insutficient in clearness and it is impossible to obtain a bronze tone which is strongly desired in this field.

For the purpose, we have made a large number of investigations and found that a clear bronze color oxide coating having excellent abrasion resistance, corrosion resistance and weather resistance can be obtained by anodizing an aluminous substrate in the above described electrolytic bath of the present invention to form an amberish gray oxide coating and then, without sealing the pores of the oxide coating, immersing the aluminous substrate in an aqueous solution of ammonium ferric oxalate to form a reddish brown oxide coating of ferric hydroxide on the above amberish gray oxide coating, whereby these two colored oxide coatings are combined to form a clear bronze color.

The fifth aspect of the present invention is a method of producing the above described clear bronze color oxide coating, which comprises anodizing an aluminous substrate in the above described electrolytic bath of the present invention in a first step to form a colored oxide coating and then immersing the thus treated aluminous substrate in 1 to 10% by weight of an aqueous solution of ammonium ferric oxalate in a second step.

In the second step of this process, when the content of ammonium ferric oxalate is less than 1% by weight, the effect of addition does not occur and substantially no reddish brown ferric hydroxide is deposited on the aluminous substrate and in order to deposit ferric hydroxide, a fairly long treating time is necessary. On the other hand, when the content of ammonium ferric oxalate is more than 10% by weight, the deposit of ferric hydroxide is too rapid and it is diificult to deposit ferric hydroxide on the aluminous substrate uniformly.

These facts have been confirmed by the following experiment.

28 aluminum was anodized in a bath comprising an aqueous solution of 10% by weight of cresolsulfonic acid and 0.6% by weight of sulfuric acid at 9 current density of 2 a./dm. and a voltage of 29 to 42 volts and a temperature of 20 C. for 50 minutes and then immersed in an aqueous solution of ammonium ferric oxalate at 60 C. by varying the content from 0.5% by weight to 11% by weight as shown in the following Table 3 for 3, 4 and 5 minutes. The colors of the resulting oxide coatings on 25 aluminum were determined with respect to Adams color diiference by a color-difference meter made by Toyo Rika Kogyo K.K. by comparing with the color of oxide coating on 28 aluminum treated with only the first step as the standard color and the obtained result is shown by NBS unit (National Bureau of Standards Unit) in the following Table 3.

TABLE 3 Percent 1 of As seen from the results in Table 3, if the immersing time in the second step increases, the color of the oxide coating formed on aluminum substrate becomes more clear. For example, when the content of ammonium ferric oxalate is 2% by weight and the immersing time is 3 minutes, the color difference in NBS units is 5.0, but in 5 minutes, the color difference in NBS units is 7.8. Furthermore, even if the content of ammonium ferric oxalate is smaller, the effect of the addition can be developed by increasing the immersion time but it has been found that the content of ammonium ferric oxalate which can form a satisfactorily colored oxide coating (more than 4.0 in NBS units) in the second step is 1 to by weight in the practical immersing time of 3 to 5 minutes.

For the above described anodization of the aluminous substrate, an alternating current or a direct current can be used as an electric source and, if necessary it is possible to use both the currents together.

The conditions of the above described second step are as follows. The concentration of the ammonium ferric oxalate is 1 to 10% by weight as described above, the treating temperature is 40 to 70 C. and the treating time is 3 to 10 minutes.

The metal sulfates to be used in the electrolytic bath of the present invention include ferric sulfate, cobalt sulfate, manganese sulfate, nickel sulfate and the like.

The following examples are given in illustration of this invention and are not intended as limitations thereof. In these examples, percent means by weight.

EXAMPLE 1 A thin plate of aluminum 1100 was used as a substrate and firstly subjected to a pre-treatment, in which the thin plate was immersed in a 5% by weight caustic soda solution for about 90 seconds and cleaned with water and a 10% sulfuric acid solution and then again cleaned with cold water to completely remove sulfuric acid from the thin plate.

The thus cleaned thin plate was anodized as the anode in an electrolytic bath containing 100 g./l. of cresolsulfonic acid and 6 g./l. of sulfuric acid in water with a direct current for about 30 minutes. In this case, the bath temperature was maintained at :3" C. and the current density was 2 a./dm. and the voltage was raised from 29 volts to 42 volts.

After the substrate was anodized in the above described electrolytic bath under the conditions as described above, a yellowish brown oxide coating having a thickness of 18 2 was obtained. Thereafter a treatment for sealing the pores was effected on the resulting oxide coating with pure boiling water for 20 minutes. Then an abrasion test was effected according to JIS H8601 and the abrasion resistance was found to be 4800 seconds.

The test 118 H8601 was effected as follows by means of the apparatus as shown in the attached figure. Namely, a sample 8 to be tested is fixed on a sample plate 7 by a set bolt 6. A feeder 1, a funnel 3 having a taper angle of 60, an inner diameter of top portion of 70 mm. and provided with a leg portion, the length of which is 50 mm. and the inner diameter of which is 50:0.1 mm. and a conduit 5 are arranged as shown in the figure, so that abrasives falling from the feeder 1, through an outlet 2, the funnel 3 and the conduit 5 strike against the sample 8 and the surface of the sample forms an angle of to the perpendicular direction an amount of 320:10 g./min. of the abrasives having a specific gravity of more than 3.16, more than 92% of silicon carbide and free carbon of more than 1.5% are dropped on the sample 8 and the time until the coating film of the sample is broken is determined and read in second.

Furthermore, a weather resistance test was effected on the colored oxide coating by means of a rfadeometer for 100 hours according to I18 L1044 and the color stability of the oxide coating was found to be very high.

Then, anodizing was carried out with the same composition of the electrolytic bath and the same conditions as described above except varying only the composition of aluminum alloy. Namely, aluminum alloys 6061 and 6063 were used as substrates and anodized for 30 minutes respectively. In this case, a dark black oxide coating having a thickness of 18a was formed on the aluminum alloy 6061, while a bronze oxide coating having a thickness of 18a was formed on the aluminum alloy 6063.

EXAMPLE 2 An aluminum 1100 previously cleaned as described in Example 1 was immersed in an electrolytic bath having the composition as shown in the following Table 4 and then the anodizing process was effected at a bath temperature of 20 C. and a current density of 2 a./dm. for 30 minutes to obtain the results as shown in Table 4. In this case, the reason why the treating time is 30 minutes is based on the fact that this time is industrially suitable for practice.

TABLE 4 Composition of electrolyte (g./l.) Thiek- Cresol- Sul- Required ness of sulfonic furic voltage. coating acid acid volts f) Color tone 25 6 35-29 15 Yellowish brown. 6 30-40 18 Do. 4 32-53 15 Do. 100 6 29-42 18 Do. 100 8 28-33 18 Do. 200 4 32-55 15 Light yellowish brown. 200 6 29-42 18 D0. 200 8 28-34 18 Do. 300 6 28-42 18 Do. 400 6 28-42 18 Light white.

As seen from the above Table 4, when the amount of cresolsulfonic acid is 25 g. and the amount of sulfuric acid is 6 g., an oxide coating is produced. Furthermore, when the amount of cresolsulfonic acid added is more than 50 g., as long as the sulfuric acid is added in an amount of more than 6 g., the degree of dissociation of the electrolytic bath is large, the dissociation voltage of the bath is low, the required electric power is small and the elevation of the bath temperature is prevented.

Moreover, it can be seen that when increasing the amount of sulfuric acid added, the degree of dissociation increases and the required voltage decreases. Namely, when the amount of cresolsulfonic acid was 100 g. and the amount of sulfuric acid was increased to 6 g., 8 g. and 10 g., the required voltage decreased to 32-53 volts, 29-42 volts and 28-33 volts, and the thickness of the resulting oxide coating increased to 15, 18 and 18 1, respectively.

EXAMPLE 3 Various aluminum alloys having the composition and shape as shown in the following Table 5 were anodized under the electrolytic conditions as shown in Table 5, respectively.

The results obtained are shown in Table 5.

ance and corrosion resistance capable of being used in practice.

Furthermore, the weather resistance was examined by TABLE 5 Composition of electrolyte (percent by weight) Current Treat- Cresol- Salfodening Thicksulsali- Sulsity time ness of Experimental Alloy tonic cylie furic (a./ (min- Voltage coating number (shape) acid acid acid dmJ) ute) (volts) Appearance Comparative:

1 2 30 42-62 Uneven cor- 2 30 d 2 30 2 30 2 30 2 40 65 Dark black 20 2 20 35-45 Yellowish brown 14 2.5 20 3860 Bronze 18 2 40 30-53 do 12 Thin plate. 2 Shaped article.

3 At maximum.

EXAMPLE 4 An aluminum substrate 53ST was immersed in a 5% caustic soda solution at 60 C. for 2 minutes to effect through degreasing and then cleaned with water. The de- 5 greased aluminum substrate was immersed in an electrolyte consisting of a aqueous solution of sulfuric acid and anodized at a liquid temperature of C. and a current density of 1.5 a./dm. for about 20 minutes to produce a porous oxide coating of about 9 on the surface of the substrate. Then the substrate was thoroughly washed with Water without sealing the pores and thereafter immersed in an aqueous solution containing 10% cresolsulfonic acid and 0.6% sulfuric acid and anodized at a current density of 2 a./dm. for 3 minutes, whereby an elegant and light yellowish brown oxide coating having a thickness of about 111w was produced on the surface of the substrate. Thereafter, the substrate was cleaned with water and the resulting yellowish brown oxide coating was subjected to a treatment for sealing pores under a steam pressure of 4 kg./cm.

EXAMPLE 5 An aluminum substrate 28 was degreased in the same manner as described in Example 4 and then immersed in an electrolyte consisting of a 3% aqueous solution of oxalic acid and thereafter, the anodizing process was effected at a current density of 1 a./dm. with superimposed direct and alternating currents for 30 minutes to obtain a porous oxide coating of about 8,41. on the substrate. Then the substrate was cleaned with water as in Example 4 and immersed in an aqueous solution containing 10% cresolsulfonic acid and 2% ferric sulfate and the anodizing process was effected at a current density of 2 a./dm. for 5 minutes to produce an oxide coating having a thickness of 11a. In this case, the color tone of the resulting oxide coating was slightly deeper than that of the oxide coating in Example 4. Then the substrate was cleaned with water and the oxide coating was subjected to a treatment for sealing the pores under a steam pressure of 4 kg./crn.

An abrasion test according to H8 H8601 and a corrosion test were effected with respect to the colored oxide coatings obtained in Examples 4 and 5. The results are shown in the following Table 6. 5

It has been found from this result that these coatings are colored oxide coatings provided with abrasion reslstmeans of a fadeometer with respect to the light yellowish brown aluminum substrate of the present invention and conventional aluminum substrate colored to yellowish brown in a conventional known dyeing method, in which an oxide coating is dyed with a dyestuff. As a result, fading was never observed in the colored aluminum substrate of the present invention even after an exposure period of 1000 hours, while in the conventional colored aluminum substrate, fading was observed after an exposure of 1000 hours.

EXAMPLE 6 An aluminum 28 plate having a purity of more than 99% was immersed in a 5% caustic soda solution at 60 C. for 2 minutes to thoroughly effect degreasing and then cleaned with water. Then the degreased plate was immersed in a 10% nitric acid solution at room temperature for 1 minute and cleaned with water.

The thus cleaned plate was immersed in a first electrolytic bath containing 100 g./l. of cresolsulfonic acid and 6 g./l. of sulfuric acid and then the anodizing process was effected under the electrolytic conditions of a liquid temperature of 20i3 C., current density of 2 a./dm. and a voltage of 20-42 volts, for 30 and 50 minutes, respectively. In this case, the plate anodized for 30 minutes is referred to as plate A and the plate anodized for 50 minutes is referred to as plate B.

These plates A and -B were immersed in a second electolytic bath containing 40 g./l. of ammonium ferric oxalate without sealing the pores and anodized at a liquid temperature of 60-70 C. and a current density of 2 a./drn. for 2 minutes, respectively, whereby a light bronze oxide coating having a thickness of 18 was produced on the surface of plate A and a deep bronze oxide coating having a thickness of 28,14. was produced on the surface of plate B. Thereafter, plates A and -B were cleaned with water and the oxide coatings were subjected to a treatment for sealing pores under a steam pressure of 4 kg./cm.

An abrasion test according to HS H8601 and a weather resistance test according to JIS K4003 were effected on the oxide coatings produced on the surfaces of plates A and B as described above, respectively. In this abrasion text, the abrasion resistance of each oxide coating is mpre than 4800 seconds and it is confirmed that these oxide coatings have an extremely excellent abrasion resistance since the abrasion resistance is practically sufficient on the order of 1000 seconds. Furthermore, in the above weather resistance test, the fading was never observed in the oxide coatings of plates A and B even after the exposure period of 1000 hours.

EXAMPLE 7 Aluminum 638 plates were degreased and cleaned with a caustic soda solution and nitric acid in the same manner as described in Example 6, whereafter the cleaned plates were anodized with the same first and second electrolytic baths under the same conditions as described in Example 6 for 50 minutes and 20 minutes, respectively. The obtained results were the same as those of Example 6.

What is claimed is:

1. Anodizing electrolytic bath for producing colored aluminum or aluminum alloy, which is an aqueous solution of 0.5 to 10% by weight of cresol sulfonic acid, 0.5 to 5.0% by weight of sulfosalicylic acid and 0.05 to 3.0% by weight of sulfuric acid or a metal sulfate in an amount equivalent to the concentration of said sulfuric acid.

2. Anodizing electrolytic bath as claimed in claim 1, wherein the metal sulfate is ferric sulfate, cobalt sulfate, manganese sulfate or nickel sulfate.

3. A method for producing a colored oxide coating on aluminum or aluminum alloy, which comprises electrolytically anodizing aluminum or aluminum alloy in an electrolytic bath which is an aqueous solution of 0.5 to 10% by weight of cresolsulfonic acid, 0.5 to 5.0% by weight of sulfosalicylic acid and 0.05 to 3.0% by weight of sulfuric acid or a metal sulfate in an amount equivalent to the concentration of said sulfuric acid.

4. A method as claimed in claim 3, wherein the metal sulfate is ferric sulfate, cobalt sulfate, manganese sulfate or nickel sulfate.

5. A method for producing a cooled oxide coating on aluminum or aluminum alloy, which comprises electrolytically anodizing aluminum or aluminum alloy in an aqueous solution of sulfuric acid or oxalic acid to form an oxide coating on the surface of said aluminum or aluminum alloy without sealing the pores of the oxide coating and then electrolytically anodizing said oxide coated aluminum or aluminum alloy in an electrolytic bath which is an aqueous solution of 0.5 to 10% by weight of cresol- 12 sulfonic acid, 0.5 to 5.0% by weight of sulfosalicylic acid and 0.05 to 3% by weight of sulfuric acid or a metal sulfate in an amount equivalent to the concentration of said sulfuric acid.

6. A method as claimed in claim 5, wherein the metal sulfate is ferric sulfate cobalt sulfate, manganese sulfate or nickel sulfate.

7. A method for producing a bronze colored oxide coating on aluminum or aluminum alloy which comprises electrolytically anodizing aluminum or aluminum alloy in an electrolytic bath which is an aqueous solution of 0.5 to 10% by weight of cresolsulfonic acid, 0.5 to 5.0% by weight of sulfosalicylic acid and 0.05 to 3.0% by weight of sulfuric acid or a metal sulfate in an amount equivalent to the concentration of said sulfuric acid and then immersing the thus treated aluminum or aluminum alloy in a 1 to 10% by weight aqueous solution of ammonium ferric oxalate.

8. A method as claimed in claim 7, wherein the metal sulfate is ferric sulfate, cobalt sulfate, manganese sulfate or nickel sulfate.

References Cited FOREIGN PATENTS 731,497 2/1943 Germany 20458 677,501 6/1939 Germany 20458 409,679 4/ 1934 Great Britain 204-58 378,521 8/1932 Great Britain 204-58 57,075 12/1939 Denmark 20458 JOHN H. MACK, Primary Examiner R. L. ANDREWS, Assistant Examiner 

